1. Field of the Invention
The present invention relates to a light guide plate capable of forming a bright and high-contrast reflection-type liquid-crystal display device, and a planer light source unit that uses the light guide plate to permit efficient utilization of light.
The present application is based on Japanese Patent Application No. Hei. 10-64311, which is incorporated herein by reference.
2. Description of the Related Art
To meet the current need for illumination equipment that enabled the viewing of a reflection-type liquid-crystal display device in the dark or other situations, the present inventors attempted the application of a front light system in which the back light used in a transmission-type liquid-crystal display device was provided on the viewing side of the liquid-crystal cell. The back light uses a light guide plate that receives incident light on a side and that allows it to emerge from either the top or the bottom surface through a light issuing means. The front light system allows the display to be viewed via the light guide plate.
However, the back light using the conventional light guide has involved difficulty in attempts for commercialization since it suffers from various problems such as low contrast in ON state, low brightness in OFF state and disturbed display.
It should also be mentioned that a back light using a light guide plate in which diffusing dots or fine asperities form a light issuing means provides only low contrast during viewing in On state and also provides low contrast and insufficient display brightness during viewing in OFF state with extraneous light such as indoor illumination; in addition, the display image that can be viewed through the light guide plate is so extensively disturbed that it lacks clarity.
A back light is known that uses a light guide plate in which a stepped prism structure consisting of inclined surfaces having a slope of 45 degrees and flat surfaces (zero degrees of inclination) form a light issuing means (as described in Japanese Patent Publication No. Sho. 62-73206). Even this back light has the disadvantage of giving low contrast and insufficient display brightness during viewing in ON state.